Method and apparatus for evaluating formation resistivity using focused annular and azimuthal electrodes

ABSTRACT

The apparatus includes an array of azimuthal current electrodes (Aaz i ) that are circumferentially spaced apart from one another on a body together with two annular guard electrodes (A2) disposed on opposite sides of the array of azimuthal current electrodes (Aaz i ). Measurement currents (Iaz i ) are emitted by the current electrodes (Aaz i ) and focusing currents are emitted by the guard electrodes (A2). Azimuthal monitor electrodes (Maz i ) are associated with respective ones of the azimuthal current electrodes (Aaz i ). Two annular monitor electrodes (M3, M4) are disposed on opposite sides of the array of azimuthal current electrodes (Aaz i ). For focusing purposes, a servo-control system controls the emission of the currents so as to substantially cancel the potential difference detected between the short circuited monitor electrodes (M3, M4) and each of the azimuthal monitor electrodes (Maz i ). In response to the measurement currents (Iaz i ), output signals (Raz i ) are generated representative of the resistivity of the formations in a plurality of directions around the axis of the borehole. A high resolution output signal (Rhr) is also generated as a function of the sum of the measurement currents (Iaz i ).

BACKGROUND OF THE INVENTION

The invention relates to well logging for evaluating the resistivity ofearth formations through which a borehole passes, and more particularlyto an electrical logging method and apparatus using electrodes.

A well logging device with electrodes that has been commerciallyavailable for many years and that is known under the name "DualLaterolog" is described in U.S. Pat. No. 3,772,589 (Scholberg). Thatdevice comprises an array of annular electrodes used for sendingelectrical measurement currents into the formations for the purpose ofmeasuring their resistivity. The measurement currents are focused in anannular zone having the form of a disk perpendicular to the boreholeaxis by means of auxiliary currents emitted by guard electrodes. Thatdevice includes measuring deep resistivity of the earth formations (LLdmode) and measuring shallower resistivity of the earth formations (LLsmode) by emitting currents at different frequencies, typically 35 Hz and280 Hz.

A drawback of that device is that its longitudinal resolution (i.e. inthe longitudinal direction of the borehole) is poor, being about onemeter. In addition, in some cases and in particular in boreholes thatare deviated or horizontal, the annular zone scanned by the devicearound the borehole comprises different layers of earth formation andgives an average measurement that has little meaning. It is thereforedesirable to obtain resistivity measurements in a plurality of azimuthaldirections around the borehole.

Logging apparatuses are known that detect the resistivity of formationsin a plurality of azimuthal directions around the axis of the borehole.Those devices seek to obtain information about fractures or about thedip of formations. For example, British patent 928 583 (BritishPetroleum Company Limited) describes an array of azimuthal measurementelectrodes distributed circumferentially around the periphery of alogging sonde. A guard electrode which surrounds the measurementelectrodes enables an auxiliary current to be emitted for focusing thecurrents emitted by each of the measurement electrodes.

In such a sonde, measurement current focusing is passive, and thisfocusing is obtained by emitting the various currents via electrodesthat are short-circuited together. Unfortunately, focusing must beparticularly effective since the measurement electrodes are disposed onthe body of the sonde and thus may be spaced-apart long distances, up toabout 10 cm, from the wall of the borehole, such spacing distances beingpossibly different in different directions when the sonde is notcentered in the borehole. A defect of this passive focusing technique isthe often inadequate quality of its focusing.

French patent FR 2 611 920 (CNRS) describes a logging sonde in whichcorrection means are proposed acting on the potential of the currentelectrodes in order to improve focusing. The sonde includes monitorelectrodes disposed at a certain distance ahead of the currentelectrodes and circuits that are responsive to the potentials detectedby said monitor electrodes to control the measurement currents. It isdifficult and complicated to make such a sonde, which requiresconcentric rings of electrodes.

SUMMARY OF THE INVENTION

An primary object of the invention is to provide a logging technique forobtaining accurate azimuthal resistivity measurements in order to detectfractures or other peripheral variations of resistivity in a borehole.

Another object of the invention is to provide a logging technique usingazimuthal electrodes enabling effective focusing of the measurementcurrents to be obtained both longitudinally and circumferentially.

Still another object of the invention is to provide a focused loggingtechnique that provides measurement of resistivity having highlongitudinal resolution.

In a first aspect of the invention, a method of evaluating theresistivity of earth formations having a borehole passing therethroughcomprises the steps of: emitting measurement currents into theformations via an array of azimuthal current electrodes that arecircumferentially spaced apart from one another on an elongate bodysuitable for being displaced along the borehole; emitting focusingcurrents by annular guard electrodes disposed longitudinally on the bodyon opposite sides of the array of azimuthal current electrodes; andgenerating output signals as a function of the measurement currents, theoutput signals being representative of the resistivity of the formationsin a plurality of directions around the borehole. The method furtherincludes the steps of: detecting the potentials that appear on azimuthalmonitor electrodes disposed on the body in the vicinity of each of theazimuthal current electrodes; detecting the potentials that appear ontwo annular monitor electrodes disposed on opposite sides of the arrayof azimuthal current electrodes; and controlling the emission of thecurrents in response to the detected potentials to focus saidmeasurement currents longitudinally and azimuthally.

Preferably, the measurement currents are focused by controlling theemission of the currents so as to maintain the potential differencebetween each azimuthal monitor electrode and the electrically connectedannular monitor electrodes at substantially zero. The output signals(Raz_(i)) are generated by determining for each azimuthal currentelectrode an azimuthal resistivity signal which is a function of theratio of the potential (vaz) detected on one of the annular monitorelectrodes divided by the current (Iaz_(i)) emitted by azimuthal currentelectrode.

In a preferred embodiment, a high resolution resistivity signal is alsogenerated as a function of the sum of the currents (Iaz_(i)) emitted bythe azimuthal current electrodes.

According to another aspect of the invention, an apparatus forevaluating the resistivity of earth formations having a borehole passingtherethrough comprises: an elongate body adapted to be moved along theborehole; an array of azimuthal current electrodes circumferentiallyspaced apart from one another on the body; two annular guard electrodesdisposed longitudinally on the body on opposite sides of the array ofazimuthal current electrodes; means for emitting measurement currentsvia the azimuthal current electrodes and focusing currents via the guardelectrodes; and means responsive to the measurement currents forgenerating output signals representative of the resistivity of theformations. Azimuthal monitor electrodes are disposed on the body in thevicinity of respective azimuthal current electrodes and two annularmonitor electrodes are disposed on opposite sides of the array ofazimuthal electrodes. Means responsive to the potentials on the monitorelectrodes control the emission of the currents in such a manner as tofocus said measurement currents longitudinally and azimuthally.

Preferably, each azimuthal monitor electrode (Maz_(i)) is surrounded byan azimuthal current electrode (Aaz_(i)).

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics and advantages of the invention appear more clearlyfrom the following description given by way of non-limiting example andmade with reference to the accompanying drawings, in which:

FIG. 1 shows a logging apparatus according to the invention comprisingan electrode sonde suspended in a borehole;

FIG. 2 shows a configuration of azimuthal electrodes used by the FIG. 1sonde; and

FIGS. 3A and 3B are circuit diagrams respectively of the downholecircuits and the surface circuits in the logging apparatus of FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to FIG. 1, a logging apparatus for evaluating theresistivity of earth formations 10 having a borehole 11 passingtherethrough comprises a sonde 12 suspended in the borehole at the endof a multiconductor cable 13. The cable 13 passes over a sheave 14 andis wound onto a winch 15 for moving the sonde 12 along the borehole. Thewinch 15 constitutes a portion of a surface unit 16.

The sonde 12 has four sections fixed end to end so as to make up anelongate body 17. The top section 20 is a sealed metal housingcontaining electrical circuits that are described in greater detailbelow. A first intermediate section 21 comprises a tubular body 22 whichcarries an array 23 of azimuthal electrodes that are circumferentiallyspaced apart from one another. A second intermediate section 24 fixed tothe bottom of the first intermediate section 21 carries annularelectrodes used for conventional Laterolog type measurements. A bottomsection 25 comprises a metal body 26 having four centralizing shoes 27adapted to bear against the wall of the borehole 11 under drive fromleaf springs 28. A measuring shoe 30 hinged to the bottom end of one ofthe centralizing shoes 27 is urged by an individual spring 31 againstthe wall of the borehole. This measuring shoe 30 is fitted withconventional electrodes for performing conventional type sphericallyfocused microresistivity measurements.

The sonde 12 includes a first array of annular electrodes forimplementing the Dual Laterolog technique described in U.S. Pat. No.3,772,589 (Scholberg). As described in that patent, the intermediatesection 24 carries a central electrode Ao, a first pair of monitorelectrodes M1, M'1 connected to each other and disposed on oppositesides of the electrode Ao, a second pair of monitor electrodes M2, M'2disposed on opposite sides of the pair M1, M'1, and a first pair ofguard electrodes A1, A'1 that are connected to each other and aredisposed on opposite sides of the pair M2, M'2. The sonde also includesa second pair of guard electrodes A2, A'2 that are connected to eachother, with the guard electrode A2 being formed by the body 22 of theintermediate section 21 and with guard electrode A'2 being formed by thebody 26 of the bottom section 25.

In the Dual Laterolog .technique, the resistivity of the formations attwo different radial investigation depths are measured by sendingelectrical currents at two different frequencies f1 and f2. For theshallow measurement (LLs mode) an alternating measurement current isemitted at a first frequency f1 (e.g. 280 Hz) by the central electrodeAo, said measurement current being focused by an auxiliary current sentbetween the pair of electrodes A1, A'1 and the pair of electrodes A2,A'2. For further details, reference may be made to the above-mentionedScholberg patent.

For the deep measurement in LLd mode, an alternating current is used ata low frequency f2 (e.g. 35 Hz) likewise emitted by the electrode Ao.This measurement current is focused by same-frequency auxiliary currentsemitted both by the electrodes A1, A'1 and A2, A'2. The auxiliarycurrents are controlled by a feedback loop circuit that maintains thepotential difference between the two pairs of electrodes M1, M'1 and M2,M'2 at substantially zero. The measurement current is thus maintained ina zone which is disk-shaped and perpendicular to the axis of theborehole. This prior technique is used by the apparatus of the inventionfor obtaining LLd type measurement signals. A portion of the circuitsshown in FIGS. 3A and 3B performs the functions required for such LLdmode measurement.

In addition to the electrodes enabling it to perform conventional LLdand LLs mode measurements, the apparatus of FIG. 1 includes an array 23of azimuthal electrodes and other electrodes shown in greater detail inFIG. 2.

FIG. 2 shows the intermediate section 21 of the sonde. The metal tubularbody 22 of this section forms an electrode A2 that has a top portion anda bottom portion. Between the two portions of the electrode A2 anisolated central section carries a pair of annular monitor electrodes M3and M4 that are electrically connected together. Between the two annularelectrodes M3 and M4, there is an array 23 of twelve azimuthal currentelectrodes Aaz_(i) that are spaced apart circumferentially from oneanother, where i is an index in the range I to 12. Each of the azimuthalcurrent electrodes is substantially rectangular in shape being elongatein the longitudinal direction, is insulated from the body, and surroundsan azimuthal monitor electrode Maz_(i). Each azimuthal monitor electrodeMaz_(i) is insulated both relative to the body and relative to theelectrode Aaz_(i) that surrounds it. It too is substantially in the formof a rectangle and it extends over nearly the entire length of theassociated current electrode.

In the embodiment described, the annular monitor electrodes M3 and M4are situated between the portions of the electrode A2 and the array 23of azimuthal electrodes. These monitor electrodes could also be situatedwithin each portion of the electrode A2 so that the top portion of theelectrode A2 extends below the electrode M4 and the bottom portion ofthe electrode A2 extends above the electrode M3.

To obtain resistivity measurements in a plurality of directions aroundthe borehole, electrical currents are emitted into the formations fromthe azimuthal current electrodes Aaz_(i). These currents are focusedlongitudinally and azimuthally by an active feedback loop system orservo-control system that includes the azimuthal monitor electrodesMaz_(i), the annular monitor electrodes M3 and M4, and the two portionsof the guard electrode A2. For longitudinal focusing, an auxiliarycurrent is emitted by the electrode A2. In addition, the system forservo-controlling the azimuthal measurement currents provides mutualazimuthal focusing between the currents.

It has been found that the passive focusing of prior art devices can behighly disturbed by the differences in electrode contact impedance. Inpreviously described apparatuses where azimuthal electrodes are used,the survey current electrodes and the guard electrodes are maintained atthe same potential for focusing the measurement currents. When theelectrodes are immersed in the fluid of the borehole, it is observedthat the contact impedances of these electrodes can be very differentfrom one another. While current is being emitted, the potential of acurrent electrode is no longer equal to the potential of the boreholefluid facing it but is at a potential that includes an error which is afunction of the product of the emitted current multiplied by the contactimpedance. This error in the potential of the current electrode maydegrade the focusing of the measurement current. This phenomenon isparticularly troublesome with a plurality of measurement currents thatare circumferentially directed around the borehole axis.

In the apparatus of FIGS. 1 and 2, emission of the azimuthal measurementcurrents Iaz_(i) is so controlled as to maintain the potentialdifference between each azimuthal monitor electrode Maz_(i) and the setof electrically interconnected annular monitor electrodes M3, M4 atsubstantially zero. The azimuthal resistivity measurements are obtainedby taking the ratios Vaz_(i) /Iaz_(i) of the potential Vaz_(i) of one ofthe annular monitor electrodes M3, M4 divided by the azimuthalmeasurement currents Iaz_(i). In the general case, each azimuthalresistivity measurement is a function of the ratio of the potentialdetected on an azimuthal monitor electrode Maz_(i). divided by thecurrent emitted by the associated azimuthal current electrode Aaz_(i).In the example described, the potential Vaz_(i) may be detected on anyone of the monitor electrodes Maz_(i), M3, or M4.

The downhole and the surface circuits of the apparatus are showndiagrammatically in FIGS. 3A and 3B.

With reference to FIG. 3A, the downhole circuits are situated insections 20 and 21. The above-described electrodes are showndiagrammatically on the right of the figure, with only one electrodeMaz_(i) and only one electrode Aaz_(i) being shown in order to simplifythe description.

An alternating current It at a frequency of 35 Hz is sent by one or moreconductors 40 of the cable 13 from the surface to the downhole sonde.This total current It is detected downhole by means of a low resistanceseries resistor 41 whose terminals are connected to an amplifier 42followed by a bandpass filter 43 centered on the frequency of 35 Hz. Thephase of the total current It is also detected by means of a phasedetector circuit 44. Downhole measurement of the total current It anddetection of its phase make it possible to ignore any distortion thatmay have been introduced by transmission along the cable 13. A portionof the total current is applied via a conductor 45 to the electrodes A2and A'2 that are electrically interconnected by very low resistancecopper bars 46. As explained below, the total current flows between thecurrent electrodes and a remote electrode B situated on the surface.

The apparatus includes circuits used for operating in LLs mode with analternating current at 280 Hz. These circuits are not shown in FIG. 3Ain order to simplify the description. The person skilled in the art willhave no difficulty in adding such circuits to FIGS. 3A and 3B given thedetailed description thereof provided in the above-mentioned Scholbergpatent. In conventional manner, these circuits provide a shallowresistivity signal which is recorded on the surface at a function ofdepth. The present description is therefore limited to describing thecircuits that operate in low frequency mode at 35 Hz.

A portion of the circuits shown in FIG. 3A is devoted to conventionalLLd mode measurement.

A feedback loop keeps the potential difference between the pairs ofmonitor electrodes M1, M'1 and M2, M'2 at substantially zero so as tofocus the measurement current emitted by the electrode Ao. The monitorelectrodes M1 and M2 are connected to a first primary winding of adifferential transformer 50, and the monitor electrodes M'1 and M'2 areconnected to a second primary winding of the same transformer. Thesecondary winding of the transformer 50 is connected to the inputs of abroad band filter circuit 51 having high gain amplification and havingits output applied to the primary winding of a transformer 52. One endof the secondary winding of the transformer 52 is connected to theelectrode Ao and the other end thereof is connected to one of theprimary winding of a measurement transformer 53 whose other end isconnected to the guard electrodes A1, A'1 which are short-circuitedtogether by a copper bar 54. The secondary of the measurementtransformer 53 is connected to an amplifier 55 whose output is appliedto a bandpass filter centered on the frequency of 35 Hz. The output ofthe filter 56 provides an alternating signal representing the current Ioemitted by the electrode Ao.

Another feedback loop short circuits the guard electrodes A1, A'1 andA2, A'2 at the frequency of 35 Hz. A pair of interconnected monitorelectrodes A1*, A'1* is disposed in the vicinity of the guard electrodesA1, A'1. Another pair of interconnected monitor electrodes is disposedin the vicinity of the guard electrodes A2, A'2. These two pairs ofelectrodes are connected to the input of a high gain differentialamplifier circuit 60 including a filter function at the frequency 35 Hzand having its output applied to the primary winding of a transformer61. The secondary winding of the transformer 61 is connected between theguard electrode A'1 and the guard electrode A'2.

One of the monitor electrodes M1 or M2, in this case M2, is connected tothe input of a measurement amplifier 62 whose other input is connectedto the armor of the cable which serves as a remote reference electrodeN. The output of the amplifier 62 is applied to a bandpass filter 63centered on the frequency of 35 Hz and whose output provides analternating signal corresponding to the potential difference Vo betweenthe electrode M2 and the electrode N.

The above-described circuits perform the functions required for the LLdoperating mode as described in the Scholberg patent.

The apparatus shown in FIG. 3A also includes circuits for generatingazimuthal resistivity signals. The short-circuited annular monitorelectrodes M3 and M4 are connected to the input of a measurementamplifier 64 whose other input is connected to the reference electrode Non the cable armor. The output of the amplifier 64 is applied to abandpass filter centered on the frequency of 35 Hz which provides analternating signal Vaz_(i) representative of the potential differencebetween the electrodes M3, M4 and the reference electrode N.

Each azimuthal monitor electrode Maz_(i) is connected to the input of ahigh gain differential amplifier circuit 68_(i) which also includes afilter function at 35 Hz. The other input of each circuit 68_(i) isconnected to the annular monitor electrodes M3, M4 and its output isapplied to a current source 69_(i) constituted by a voltage-currentconverter. The output current from the source 69_(i) is applied betweenthe guard electrode A2 and the azimuthal current electrode Aaz_(i)corresponding to the monitor electrode Maz_(i) under consideration. Thisloop controls each of the azimuthal currents Iaz_(i) in such a manner asto maintain the potential difference between M3. M4 and thecorresponding azimuthal monitor electrode at zero.

The output signal from the circuit 68_(i) is also applied to ameasurement amplifier 70_(i) followed by a bandpass filter 71_(i)centered on the frequency of 35 Hz so as to provide an alternatingsignal Iaz_(i) representative of the measurement current emitted by theazimuthal electrode Aaz_(i). As shown in dashed lines in FIG. 3A, theapparatus has twelve identical channels providing the measurementcurrents Iaz_(i). The signals Io, Vo, Vaz_(i), It, and the twelvesignals Iaz_(i) are applied to a multiplexer 73 whose output is appliedto a variable gain amplifier 74 and then to an analog-to-digitalconverter 75. The digital outputs from the converter 75 are applied to adigital processor circuit 76 constituted by a digital signal processor(DSP) programmed to perform a phase sensitive detector (PSD) functionand a lowpass filter function. The phase reference required for thedetector function comes from the circuit 44 so as to be synchronizedwith the total downhole current It. The processor circuit 76 alsoprovides a control signal to the variable gain amplifier 74 so as toreduce the dynamic range of the input signals to the analog-to-digitalconverter 75.

The multiplexed digital signals representing the amplitudes of thecurrents or voltages Io, Vo, Vaz_(i), It and Iaz_(i) are applied toremote measurement circuits 77 for modulating and transmitting thesignals to the surface via the cable 13.

As shown in FIG. 3B, the downhole signals are received and demodulatedat the surface by a remote measurement circuit 80 and they are theninput to a computer 81 which may be a Microvax microcomputer as sold byDigital Equipment Corporation, for example. The computer 81demultiplexes the signals Io, Vo, Vaz_(i), It, and Iaz_(i), and itcalculates formation resistivity signals Ra, Rs, Raz_(i), and Rhr, usingthe following equations:

    Ra=k.sub.1 ·Vo/Io

    Rs=k.sub.2 ·Vo/It

    Raz.sub.i =k.sub.3 ·Vaz.sub.i /Iaz.sub.i

    Rhr=k.sub.4 ·Vaz.sub.i /εIaz.sub.i

in which k₁, k₂, k₃, and k₄ are predetermined constants that depend onthe geometry of the downhole sonde. Ra is the LLd type deep resistivitymeasurement; Rs is the resistivity measurement of the surroundingformations; Raz_(i) are twelve azimuthal measurements of resistivityaround the borehole; and Rhr which corresponds to a mean of theconductivity 1/Raz_(i) is a measure of formation resistivity having highlongitudinal resolution.

These various resistivity signals are recorded as a function of depth ina recording apparatus 82 which may comprise an optical recorder and amagnetic recorder. Prior to recording, the signals Raz_(i) and Rhr aredepth corrected in conventional manner to take account of the fact thatthey are obtained on the downhole sonde at a different depth from thesignals Ra and Rs.

A 35 Hz current source 83 delivers the total current It via one or moreconductors of the cable 13 and a return electrode B placed on thesurface of the ground. The current It is controlled by means of avariable gain amplifier which receives a control signal from thecomputer 81. The control signal is designed to minimize the dynamicrange of the measurement signals. One example of minimizing is given inthe Scholberg patent which maintains the product Vo·Io constant.

The embodiment described above may naturally be varied or improved innumerous ways while remaining within the scope of the invention asdefined in the following claims. In particular, the array of azimuthalelectrodes may be combined with other logging apparatuses focused byannular electrodes, such as, for example, a Laterolog 3, a Laterolog 9,or a spherical focusing type apparatus.

We claim:
 1. A method of evaluating the resistivity of earth formationshaving a borehole passing therethrough, the method comprising the stepsof:emitting measurement currents into the formations via an array ofazimuthal current electrodes circumferentially spaced apart from oneanother on an elongate body suitable for being displaced along theborehole; emitting focusing currents by annular guard electrodesdisposed longitudinally on the body on opposite sides of said array ofazimuthal current electrodes; detecting the potentials that appear onazimuthal monitor electrodes disposed on the body in the vicinity ofeach of said azimuthal current electrodes and detecting the potentialsthat appear on two annular monitor electrodes disposed on opposite sidesof said array of azimuthal current electrodes; controlling the emissionof the currents in response to said detected potentials to focus saidmeasurement currents longitudinally and azimuthally; generating outputsignals as a function of said measurement currents; and using saidoutput signals to evaluate the resistivity of the formations in aplurality of directions around the borehole.
 2. A method according toclaim 1, wherein said step of controlling the currents includes thefollowing steps:detecting potential differences between each of saidazimuthal monitor electrodes and said annular monitor electrodes thatare connected together; and controlling the currents emitted by each ofsaid azimuthal current electrodes so as to maintain said detectedpotential differences at substantially zero.
 3. A method according toclaim 1, wherein said step of generating the output signals comprisesgenerating for each azimuthal current electrode an azimuthal resistivitysignal which is a function of the ratio of the potential detected on oneof said monitor electrodes divided by the current emitted by said eachazimuthal current electrode.
 4. A method according to claim 3, furtherincluding the step of generating an additional output signal which is afunction of the sum of the currents emitted by said azimuthal currentelectrodes.
 5. Apparatus for evaluating the resistivity of earthformations having a borehole passing therethrough, the apparatuscomprising:an elongate body adapted to be moved along the borehole; anarray of azimuthal current electrodes circumferentially spaced apartfrom one another on said body; two annular guard electrodes disposedlongitudinally on said body on opposite sides of said array of azimuthalcurrent electrodes; a plurality of azimuthal monitor electrodes eachdisposed on said body in the vicinity of a respective azimuthal currentelectrode; two annular monitor electrodes disposed on opposite sides ofsaid array of azimuthal current electrodes; means for emittingmeasurement currents via said current electrodes and focusing currentvia said guard electrodes; means responsive to the potentials on saidmonitor electrodes to control the emission of said currentslongitudinally and azimuthally; and means responsive to the measurementcurrents for generating output signals evaluating the resistivity of theformations.
 6. Apparatus according to claim 5, wherein each of saidannular monitor electrodes is included in one of said guard electrodes.7. Apparatus according to claim 5, wherein each of said azimuthalmonitor electrodes is surrounded by one azimuthal current electrodes. 8.Apparatus according to claim 7, wherein each of said azimuthal currentelectrodes is substantially in the form of a rectangle that is elongatein the longitudinal direction.
 9. Apparatus according to claim 8,wherein each of said azimuthal monitor electrodes is substantiallyrectangular in shape and elongate in the longitudinal direction. 10.Apparatus according to claim 5, wherein said array of azimuthal currentelectrodes comprises twelve azimuthal current electrodes that areuniformly spaced apart around the periphery of the body.
 11. Apparatusaccording to claim 5, further comprising means for generating a meanoutput signal as a function of the sum of said measurement currents. 12.Apparatus for evaluating the resistivity of earth formations having aborehole passing therethrough, the apparatus comprising:an elongate bodyadapted to be moved along the borehole; an array of azimuthal currentelectrodes circumferentially spaced apart from one another on said body;two annular guard electrodes disposed longitudinally on said body onopposite sides of said array of azimuthal current electrodes; aplurality of azimuthal monitor electrodes each disposed on said body inthe vicinity of a respective azimuthal current electrode; twoelectrically connected annular monitor electrodes disposed on oppositesides of said array of azimuthal current electrodes; means for emittingmeasurement currents via said current electrodes and focusing currentvia said guard electrodes; means for detecting the potential differencesbetween each of said azimuthal monitor electrodes and said annularmonitor electrodes; means for controlling the emission of saidmeasurement currents in such a manner as to maintain said detectedpotential differences at subtantially zero; and means responsive to themeasurement currents for generating output signals evaluating theresistivity of the formations.
 13. Apparatus according to claim 12,wherein said means for generating output signals comprises:means fordetecting the potential difference between one of said monitorelectrodes and a reference electrode; means for detecting themeasurement currents emitted by each of said azimuthal currentelectrodes; and means for generating azimuthal resistivity signals eachof which is a function of the ratio of said detected potentialdifference to said detected measurement current emitted by said eachazimuthal current electrodes.
 14. Apparatus according to claim 13,further comprising means for generating a mean output signal as afunction of the sum of said detected measurement currents.